36 GENERAL VIEW AND BASIS OF SCIENTIFIC RESEARCH. 



mental excitement of the brain, by memory and associative 

 suggestion, &c. ; but how it is produced we do not yet know. 

 Frequently, when mental action is strong, the head be- 

 comes suddenly hot and the feet cold. The seat of per- 

 ception and ideas is believed to be in the grey cortical 

 nervous matter of the convolutions of the cerebrum. ( By 

 the study of physiology it has been placed beyond doubt 

 that the nerve-cells, which exist in countless number 

 about 600,000,000 according to Meynert's calculations 

 in the grey matter spread over the surface of the hemi- 

 spheres, are the nervous centres of ideas.' l All mental 

 action appears to depend upon and to produce physical 

 cerebral impressions, and sensations originally precede 

 ideas. 



According to the doctrine of ' relativity,' 2 we only feel 

 or perceive a change of state; a thought includes a percep- 

 tion of relation of similarity or difference. The degree of 

 conscious impression made upon our senses or perceptive 

 powers depends upon their immediately previous state. 

 Cold water feels more cold to a hand which has been pre- 

 viously warmed than to one already cool, because in the 

 former case there is a greater degree of nervous change. 

 The greater and more sudden also the degree of nervous 

 or mental alteration, the stronger the sensation or mental 

 impression ; and we are only conscious of the stronger and 

 more sudden sensorial and cerebral changes, because our 

 senses and perceptive powers are not sufficiently refined to 

 enable us to feel or perceive the more gradual or more 

 minute ones. It is the most conspicuous differences which 

 most impress us. The term consciousness is usually taken 

 to mean sensibility in general ; all our primary con- 



1 Maudsley, Physiology of Mind, p. 259. 



2 See Bain, Sense* and Intellect, 2nd edit. p. 9. 



